"The stream of moisture coming down from the north-west into eastern Australia, across from the Indian Ocean, is a critical element in the production of rainfall," Mr Bennett said.

"In WA, a good rainfall event is one where we have a cold front moving up from the Southern Ocean interacting with a feed of moisture coming down from the Indian Ocean and producing widespread cloud and heavy rainfall.

"If you have a positive Indian Ocean Dipole and you have cooler than normal ocean temperatures, then the amount of moisture available is reduced compared to if you have above average temperatures as you would get in a negative IOD."

El Nino threat decreases for east coast

The BOM's outlook for June through to August appears grim for the already parched agricultural areas across Australia.

It shows winter is likely to be drier than average for much of Queensland, New South Wales, Victoria, south-eastern parts of South Australia, northern parts of Tasmania, the Northern Territory, and northern and far south-western parts of WA.

But while the outlook suggests an increased chance of a positive IOD event forming during winter, it also shows a gradual weakening of the El Nino-like pattern in the tropical Pacific Ocean, which typically brings drier than average conditions for eastern Australia.

The Bureau has slashed the chances of El Nino occurring from 70 per cent (an alert level) to 50 per cent (a watch level), which is still double the normal likelihood.

Mr Bennett said unlike El Nino, the IOD tended to have a widespread impact across Australia.

"With an El Nino being over in the Pacific Ocean, there's less of an impact in Western Australia than the eastern states, whereas the IOD can have an impact right across the whole of southern and central Australia," he said.

The IOD usually impacts Australia from May or June onwards, peaking in about July or August, before decreasing from October as the southern hemisphere monsoon kicks in.

But a positive IOD does not guarantee dry weather.

Just like El Nino and La Nina in the Pacific Ocean, the IOD is a background influence on the weather and individual heavy downpours can still occur.

When the Indian and Pacific oceans conspire

Australians can count their blessings that they aren't dealing with both an El Nino and a positive IOD at the same time.